Throughout the following background description, the field of electronic systems for automotive, in particular motor car, applications will be considered by way of non-limiting example. Thus, it will be appreciated that other applications may exist.
As is well known, a number of electronic and electromagnetic devices installed on motor cars have at least one inductive load. Such devices include, for example, relays of various types, motor driver devices, and fuel injection control devices. References will be made hereinafter to fuel injection control devices, by way of example.
Electronic fuel injection devices operate on the principle of opening the fuel path through an electronically controlled valve. The valve movement is controlled by means of a magnetic field generated by an electromagnet, which can be illustrated schematically by an inductor wound around a core and having a drive current flowing therethrough. By regulating the current flowing through the inductor, the injector can be controlled to open and close. This sets the amount of fuel being injected. In such devices, the inductive load has a terminal maintained at a fixed voltage and another terminal connected to a preferably integrated, control circuit operative to drive the flow of current through the load.
There exist two arrangements, wherein the fixed voltage terminal of the load is respectively connected to the power supply line and connected to ground. In the former arrangement, the load is driven through its low voltage terminal, in a so-called low-side configuration commonly employed in injection devices, whereas in the latter, a so-called high-side driver configuration is used. The control circuit includes an active element, which may be a drive transistor, usually a power one, acting as a switch to alternately force the current flow through the inductor and cut it off. The active element is controlled by a drive signal generated within the circuit.
As is known, the driving of highly reactive loads, as inductive loads are, generally causes some problems during the transients. When the flow of current through an inductor is cut off, the voltage across the inductor rises instantaneously, i.e. an overvoltage is created which may be positive or negative depending on the particular circuit configuration. This overvoltage is due to the energy, previously stored into the inductor during the charging phase and originated by the forced flow of electric current through the inductor, staying constant even after the current flow is cut off. Thus, an induced electromotive force is generated which tends to keep the current at the value it attained during he power-up period, i.e. during the charging phase. Since the load has one terminal held at a fixed electric potential, the increase of potential will take place on the other terminal, that is the connection terminal to the control circuit.
For the device to operate properly, the amplitude of the voltage peak must be a limited one. Otherwise, the peak could cause the junctions in the control circuit semiconductor elements, or in any other devices linked to the control circuit, to break down. Where the circuit is a monolithically integrated one, the overvoltage may also trigger on parasitic transistors and possibly cause the device to break down. As those skilled in the art know well, the measures to be taken in order to reduce this overvoltage include the provision of some means effective to dissipate the energy stored within the inductor, that is, effective to make a discharge current from the inductor to recycle. The energy stored in the inductive load is dissipated by having current passed out through pre-arranged elements, usually power elements.
In prior approaches, the recycling circuit also functions to regulate the voltage rise. As the current is being recycled, the voltage at the inductive load reaches indeed a maximum which is preset by the recycling means itself. This voltage limiting effect is known as "clamping" in technical literature. The voltage value clamped at the above-mentioned maximum remains constant for some time, to then decrease in absolute value, simultaneously with the current, down to a zero value which corresponds to a fully discharged inductive load. The duration of the discharge phase, i.e. the so-called discharge time, is dependent on the highest voltage value attained on the inductor.
The discharge time is of special importance, and should be accurately controlled. For example, with car-mounted devices, it is on the basis of this time that logic control circuits operatively connected to them are correspondingly calibrated. Furthermore, it is highly important for a fuel injector that the control current flowing through the load can be cut off within the shortest possible time, so that the injector closing time can be made short, such as to meter out accurately the amount of fuel, and minimize the waste of fuel upon the injector closing.
The discharge phase regulation is obtained by suitably dimensioning the recycling circuit such that the maximum voltage value can be selected to correspondingly suit. A well known class of circuit designs, to which the present invention is more specifically related, provide for recycling of the current through the very transistor which drives the inductive load and is inherently capable of withstanding the passage of the discharge current peak, it being usually a power element. A recycling regulating and driving circuit is connected between the load terminal intended for connection to the control circuit, i.e. to the drive transistor, and a control terminal of the transistor. This circuit controls the voltage value at the load and automatically turns on the transistor upon that voltage reaching a predetermined maximum. For an injector, the maximum voltage value is usually on the order of tens of volts, a typical value being 70 volts, for instance. A first prior circuit design provides for connection of generally one or more chains of n zener diodes between the control terminal of the drive transistor and the inductive load. The zeners are arranged to set the maximum voltage at the connection node to the load at a value V.sub.max equal to nV.sub.z, V.sub.z being the zener voltages, less the voltage drop between the control terminal and the connection node, e.g., a VGS for a MOS type of drive transistor.
An improvement on the above prior approach additionally includes one or more bipolar transistors in the recycling circuit. In this case, the maximum voltage is determined by the combination of the zener voltages and the voltage drop base-emitter of the additional transistors and, accordingly, is preset as a combination of a larger number of voltage drops than in the previously described circuit. Where good temperature compensation is required for each selected value of maximum voltage, still more sophisticated precision designs are used which have good stability. For example, one prior circuit design uses a recycling regulating circuit which includes a voltage divider and a comparator. A circuit of that type is described, for example, in European Patent Application EP-0622717 by this Applicant.
To better appreciate the objectives of the present invention, it should be considered that--in general and regardless of the recycling regulating circuit being used--the output voltage, i.e., the voltage at the connection node to the inductive load, would not attain, at once and monotonically, the zero value that corresponds to full discharge, at the end of the recycling. A time period elapses from the moment when the drive transistor starts to power down to the moment when it is fully of. During this period, the voltage, and more generally the output electrical quantities, follow an oscillating trend.
It should be observed in this respect that a parallel circuit of the LRC type--L, R and C being overall values of inductance, resistance and capacitance, respectively--operates as a damped harmonic oscillator. This effect is due essentially to the reactive characteristics of the inductor and to conservation of energy, with consequent periodical transfer thereof between the inductance and the capacitance. When no more electromotive force is applied, i.e., during the circuit power-down phase, a more or less marked oscillation is established which depends on the circuit parameters. Of particular significance is a so-called factor of merit, Q, which is the ratio of the total stored energy to the energy dissipated during one oscillation period. For such circuits, it is Q=(.sub.o RC=R/(.sub.o L, where (.sub.o =1/ is a circuit constant having the dimensions of an angular frequency. Therefore, Q=R also is constant. The lower the Q-factor, the greater the oscillation damping.
In the instance of a recycling regulating circuit for an inductive load, the oscillating trend at the end of the recycling is due to the concurrent presence, on the connection node to the load, of an inductive load having its own inductance value, and an equivalent output capacitance which is due primarily to parasitic capacitances as seen from that connection node. For example, for a MOS drive transistor, the equivalent capacitance is substantially equal to the drain capacitance C.sub.D, whose value is high compared to the other parasitic capacitances. The inductance and capacitance, along with the equivalent resistance due essentially to the drive transistor being powered down, form a parallel circuit of the LRC type. The oscillation is sustained with a factor of merit, Q', which is predetermined by the circuit parameters selected for a particular application.
Shown in FIG. 1 is a simulation of the output voltage for a conventional control circuit for recycling the discharge current. The output voltage Vout is plotted vs. time. The circuit behavior is illustrated starting off with the recycling phase, wherein the output voltage is constant and has a predetermined value V.sub.max (clamping voltage), because of the clamping effect described previously. At a time t0, the voltage starts to drop at a fairly fast rate, depending in any case on both the clamping value V.sub.max and the magnitude of the inductive load, which, as known, can be schematically represented by an inductor L in series with a resistor R, and depending in particular on the time constant LR. The drive transistor starts to power down at a time t1 whereat the output voltage and correspondingly the current being recycled through the drive transistor reach a sufficiently low value. Due to the aforementioned effect, the voltage V.sub.out rises again from the time t1 to produce the oscillating trend. As shown in FIG. 1, it is only at a time t2 that the voltage attains a stationary value and therefore the oscillating phase is terminated, whose duration and magnitude are dependent on the circuit parameters, and in particular on the Q'-factor.
Due to this oscillation, the time to full discharge is further extended, which time should be kept as short as possible, as previously explained. Also, the interval between t1 and t2 represents a critical range for good circuit operation and is often a cause of undesired and uncontrollable behavior.
It should be first considered that in control devices for fuel injectors, the connection node to the inductive load usually represents a so-called alarm terminal or pin, i.e. protection circuits particularly for the drive transistor, are connected to it. Such circuits generate alarm signals, indicating a malfunction in the circuit and which are based on the voltage value at the node. Since this voltage value extends outside the safe range due to the oscillation shown, erroneous alarm signals may be generated accidentally, such as open-loop or open circuit signals, or vice-versa short-circuit signals. As a result, the device operation may be shut down.
In addition, an analysis of the behavior of the inductive load has shown that the new flow of current through the load during the oscillating phase can cause the injector valve to slightly open unexpectedly, causing a waste of fuel. Thus, what is needed is a simple recycling circuit which can drive an inductive load to the off state while suppressing the previously described oscillation during the final phases of the recycling.
What is also needed is such a circuit adaptable to conventional recycling circuits, such as circuits including zener diodes.